1. Field of the Invention
The present invention relates to a laser irradiation device for irradiating a medium with a laser beam modulated by Grating Light Valve, and an image recorder for recording an image on a recording material using a laser beam modulated by the laser irradiation device.
2. Description of the Background Art
Grating Light Valve is such a light valve that thousands of narrow, laterally arranged reflecting plates (or ribbons) are moved using an electrostatic force to cause diffraction which in turn is used to modulate a light beam. The Grating Light Valve functions as a plane mirror when all of the ribbons are coplanar, and as a reflective diffraction grating when alternate ones of the ribbons are moved down a distance equal to one quarter of the wavelength of a laser beam. The “Grating Light Valve” or “GLV” is trademarked and marketed by Silicon Light Machines (Sunnyvale, Calif., U.S.A.). Grating Light Valve™ is considered to function as a plane mirror when all of the ribbons are coplanar as mentioned above. It has been proved that light reflected from Grating Light Valve™ has a characteristic resulting from its structure.
More specifically, studies of the present inventor have shown that the reflectance of Grating Light Valve™ depends on the polarization of a laser beam incident on Grating Light Valve™, and that the reflectance of Grating Light Valve™ is maximum when the polarization of the laser beam is the same as a direction in which the ribbons are arranged (or is perpendicular to the longitudinal direction of the ribbons) and is minimum when the polarization direction of the laser beam is perpendicular to the direction in which the ribbons are arranged (or is the same as the longitudinal direction of the ribbons).
An image recorder which employs Grating Light Valve™ uses the diffraction grating to reflect and modulate a laser beam, thereby recording an image on a recording material.
In recent years, image recorders which use a technique (Computer-To-Plate or CTP technique) for directly recording an image on a printing plate have often employed a heat-sensitive recording material (or a thermal plate). Since the thermal plate has a lower sensitivity than a conventional silver halide based photosensitive material, a laser source having high power of 20 to 60 W is used.
For recording on the thermal plate, a laser source which emits a laser beam having a peak wavelength of 830 nm is often used and, in particular, used as a standard in the fields of printing and prepress.
Studies of the present inventor have also shown that the spectral reflectance of Grating Light Valve™ has a shoulder around 830 nm when the reflectance strongly depends on the wavelength.
Further, if a laser source which emits a laser beam having a peak wavelength of 830 nm is used, the reflectance of Grating Light Valve™ may be different from one another due to manufacturing process variations.
Furthermore, insufficient reflectance of Grating Light Valve™ leads to accordingly more energy of the laser beam absorbed as heat by Grating Light Valve™. This causes Grating Light Valve™ to be damaged by the absorbed heat energy.